Numerous problems face the designer of a shrouded gas turbine blade as a result of the high heat and high speed environment in which the shrouded blade must operate. Vibration damping, creep curling, bending stresses, contact stress wear, shroud misalignment and dynamic effects are just a few of the demons facing the designer. And, as if these design problem were not enough, in airborne applications excess weight in itself is also a penalty.
Much attention has been paid in the prior art to improving the damping and bending strength of shrouded blades. However, one area where further improvement is needed is the reduction of contact-related wear between adjacent shrouded blades.
A shrouded rotor blade assembly typically comprises a plurality of airfoil blades extending radially from a rotor having a central axis, and a shroud portion which, as an assembly, forms an annulus around the axis and circumscribing all or a portion of the blades. Throughout this specification and the attached claims, the term “generally perpendicular” is used to refer to the angle of intersection of the annular segmented shroud with the radially-extending blades, and the term “generally planar” is used to refer to the annular planar section (or a segment portion thereof), rotated about the central axis point. Examples of such configuration for shrouded blades are common in the prior art, as shown in U.S. Pat. Nos. 3,576,377, and 4,243,360 for example. In contrast from this typical configuration, FR 1,252,763 in one embodiment, for example proposes a non-annular shroud arrangement which extends acutely (i.e. not generally perpendicularly) from the blades.
Typical prior art shrouded turbine blades generally have a shroud having opposed bearing or contact faces which may be shaped to facilitate interlocking of adjacent shrouds. These shrouds may include different variations in thickness, such as, for example, stiffening rails used to reduce centrifugal deflection, and gradual changes in thickness across the width of the shroud used to reduce bending stresses in the shroud. These features, however, come at the price of increases in shroud weight.
In use, fretting can occur on contract surfaces of abutting turbine shrouds, which is of course undesirable. Prior art such as U.S. Pat. Nos. 3,576,377, 4,822,248 and 6,164,916 teach that the wear resistance of the contact faces may be improved by the introduction of special wear resistant coatings or inserts. While perhaps effective, these solutions introduce manufacturing steps and materials, and therefore cost and reliability issues as well. Further improvement is accordingly needed to improve the contact wear resistance of turbine shrouds.